1. Field of the Invention
The present invention generally relates to an optical wavelength controlling method and the system thereof; and especially relates to an optical wavelength controlling method of an optical wavelength division multiplexing (WDM) transmission system, and a system thereof.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of an optical wavelength division multiplexing (WDM) transmission system. Data to be transmitted are provided in two or more streams to corresponding transmission data processing units 111, through 11n of an optical transmitting unit 10 from an external source. The transmission data processing units 111 through 11n carry out processes of format conversion, rate conversion, etc. The processed transmission data, which are electrical signals, are converted into corresponding optical signals by corresponding electric-to-optical conversion units 121 through 12n. Then, the optical signals are amplified by corresponding optical amplifiers 131 through 13n, and distribution of the optical signals is compensated for by corresponding distribution compensating units 141 through 14n. Optical intensities of the optical signals are set to the same level by corresponding variable attenuators 151 through 15n. An adder 16 multiplexes the level-adjusted optical signals, making a WDM signal, and the WDM signal is sent out to an optical transmission line 17. One or more optical repeaters 18 are provided on the optical transmission line 17 as required.
The WDM signal arrives at a divider 21 of an optical receiving unit 20, and is divided into optical signals of two or more wavelengths by the divider 21. Then, the optical signals are amplified by corresponding optical amplifiers 221 through 22n. Intensities of the optical signals are made equal to each other by corresponding variable attenuators 231 through 23n. Distribution of the level-adjusted optical signals is compensated for by corresponding distribution compensating units 241 through 24n, and the optical signals are converted into electric signals by corresponding optical-to-electrical conversion units 251 through 25n. A format conversion, rate conversion, etc., of the electric signals are processed by corresponding received data processing units 261 through 26n, and the processed electric signals are output.
Optical wavelength division multiplexing (WDM) transmission systems are required to be capable of transmitting an increasing amount of information. The requirement can be met by expanding the signal wavelength band and improving use efficiency of the wavelength band. In order to improve the use efficiency of the wavelength band, development of a modulation/demodulation method with a narrow signal spectrum spread, and studies into systems with narrow channel spacing (wavelength interval) are being undertaken.
Here, in a system with narrow channel spacing, degradation of the transmission quality due to crosstalk from an adjacent channel tends to occur. Further, since the cycle of a pass band of an add/drop device becomes shorter when the channel spacing becomes narrower, the form of the pass band property of the add/drop device also becomes less. For example, a 3 dB width of the pass band property becomes smaller, which causes the add/drop device to reduce the spectrum of an optical signal, and the optical signal is distorted, degrading the transmission quality. For this reason, it is necessary to control the wavelength of each optical signal with high precision without the signal drifting from its assigned wavelength.
Patent References 1 and 2 disclose a technique of evaluating transmission quality (Q value, and an error rate) of an optical wavelength signal, and adjusting the optical wavelength so that the transmission quality is optimized.
Patent Reference 3 discloses a technique of evaluating the transmission quality (an error rate) of an optical wavelength signal, and adjusting the optical wavelength so that the transmission quality is optimized.
[Patent reference 1] JPA, 2000-59308
[Patent reference 2] JPA, 8-321805
[Patent reference 3] JPA, 10-163971
[Non-Patent Reference 1] Furukawa Electric Jiho, July, Heisei 15, “Highly Reliable 40-mW, 25-GHz×20-ch Thermally Tunable DFB Laser Module Integrating Wavelength Monitor” by Tatsuya Kimoto, Tatsushi Shinagawa, Toshikazu Mukaihara, Hideyuki Nasu, Shuichi Tamura, Takehiko Numura, and Akihiko Kasukawa, July 2003.